The present invention relates to the protection of electrical circuits and, more particularly, to the detection of electrical faults of the type known as arcing faults in an is electrical circuit, and more particularly still to arcing fault detection in aircraft wiring.
Aircraft power systems have historically differed from ground based power systems in several ways. The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protector.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and cause the conductor to reach an unacceptable temperature.
The need for arc detection in aircraft has become increasingly clear. For example, wire arcing may be a factor in some aircraft fires. Past responses to aircraft fires have been to increase the flame retardant properties of wiring and other interior components of aircraft. Standard overcurrent devices used in circuit breakers respond to the heating effect of current in a resistive wire to xe2x80x9cthermal tripxe2x80x9d the breaker, but these do not respond to the sputtering arc currents which cause intense arc heating and fire.
We propose a better approachxe2x80x94to stop the arc when it happens rather than wait for a fire to start or for a circuit breaker to thermal trip.
Until recently, such arc detection capability has not been available in circuit breakers or relays. Arc detection has been available for 60 Hz residential, commercial or industrial systems, but has not heretofore been resolved for 400 Hz aircraft wiring systems. In addition, most aircraft circuits do not have the neutral return conductor found in 60 Hz systems. This prevents the use of differential detection of ground faults on most aircraft branch circuits. A standard aircraft circuit breaker contains bimetals and/or magnetic solenoids which provide an inverse time response to current. Arcing fault detection is not provided by these devices. Aircraft arc detection is not possible using arc detectors designed for 60 Hz circuits for several reasons. For example, 60 Hz arc detectors partly respond to ground fault which is not possible on standard aircraft branch circuits. Also, the methods used at 60 Hz cannot be automatically extended to cover a power frequency range as high as 400 Hz.
Circuit breakers have historically been the preferred protection for aerospace wiring. Present designs are based on technologies that are up to 40 years old. Advancements in electrical circuit protection introduced by the residential and commercial industries have been slow finding their way into aerospace applications. Ground Fault Circuit Interrupters (GFCI) for personnel protection have been available in the home since the early 1970""s. Under ideal conditions, GFCI can detect phase to ground arcs as low as six milliamps, but cannot detect series arcs or improve line to neutral fault trip times.
Arc Fault detection technologies are a new and exciting innovation in circuit protection in the U.S. We have found that Arc Fault Circuit Interrupters (AFCI) can be designed to detect a series or parallel arc, as well as line to neutral arcs by xe2x80x9clisteningxe2x80x9d for the unique signatures which arcs generate. We have found that AFCI can detect arc currents well below the trip curves of today""s Mil-Spec aircraft circuit breakers. This enhanced detection capability may provide improved protection from arcing conditions onboard aircraft.
An arc fault circuit interrupter is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.
Aircraft circuit breakers have historically been the best available protection for aerospace wiring. Today""s design standards are based on technologies that are up to 40 years old. In aircraft/military type breakers, the protection is provided in two ways. Short circuit currents operate a magnetic trip latch, while overload currents operate either a bimetal trip latch or hydraulic damped magnetic plunger. The xe2x80x9cinstantaneous tripxe2x80x9d is the high current magnetic trip action found on some but not all aircraft breakers. The time to trip during an overload is determined by the time it takes to heat a bimetal to the temperature that delatches the breaker. The more current that heats the bimetal, the shorter the time it takes to trip the breaker. A hydraulic-magnetic style of breaker contains a magnetic slug sealed in fluid which moves to a trip position in response to the square of the current. These circuit interruption devices are selected by aircraft design engineers to protect the aircraft wiring from overheating or melting. During arcing faults these currents are often small, short in duration and well below the over current time protection curve designed into these breakers. Recent events have brought these limitations in design and function to the forefront. xe2x80x9cElectrical arcing failurexe2x80x9d as the ignition source, has been suspected in several recent airline disasters.
We have discovered a way in which Arc Fault Circuit Interrupter (AFCI) technology can be applied to Alternating Current (AC) and may be applicable to Direct Current (DC) electrical power systems on aerospace vehicles. AFCI technology incorporates electronic circuits that can detect the arc signature, and differentiate it from normal load arcing (motor brushes, switch and relay contacts, etc.).
Arcing in a faulted AC circuit usually occurs sporadically in each half cycle of the voltage waveform. The complex arcing event causes sputtering arc""s that vary the current from normal load patterns. The precurser to the arc may be a high resistance connection leading to a xe2x80x9cglowing contactxe2x80x9d and then a series arc, or a carbon track leading to line-to-line or parallel arcing. In a home circuit breaker equipped with Ground Fault Circuit Interrupter (GFCI), a carbon or moisture track can be detected early if the short is to ground. In many aircraft circuits, the neutral conductor is not available to complete the necessary ground fault detection circuit and GFCI protection is not possible. With the introduction of AFCI breakers, protection of arcing shorts from line-to-line, not involving ground, can also be detected and interrupted.
In our arc fault interrupter, the additional electronic devices monitor both the line voltage and current xe2x80x9csignatures.xe2x80x9d In a normal operating circuit, common current fluctuations produce signatures which should not be mistaken for an arc. Starting currents, switching signatures and load changes (normal or xe2x80x9cgood arcxe2x80x9d events) can be digitally programmed in the AFCI as normal signatures waveforms. Deviations or changes from these xe2x80x9cnormalxe2x80x9d signatures are monitored by electronic circuits and algorithms to determine if arcing is occurring. When these arc fault signatures are recognized, the circuit is interrupted and power is removed. The speed of this detection as well as the arc magnitude can be programmable parameters at the time of manufacture. The particular signatures identified as arcs are part of the proprietary arc fault technology of Square D Company.
Commercial, UL approved AFCI circuit breakers are available commercially. These are now in the NEC and will be required in home bedroom circuits 2002. Since the electrical loads in residential circuits can vary widely, they will be designed to allow for almost an infinite combination of electrical loads. Their AFCI programming is combined with GFCI as well as magnetic and thermal overload components. They are designed to form fit and function in place of standard residential circuit breakers.
We have found that in principle, design and programming of AFCI devices for aerospace applications can be simpler than those of residential devices. The homeowner expects to be able to plug any load into an outlet without nuisance tripping from an AFCI. Contrast this with commercial aerospace applications where the loads on a given circuit are fixed by design. The load on each breaker is carefully planned. Deviations from the original OEM specifications require special analysis and FAA approval. Fixed loads coupled with standardized wiring practices, connectors and certifications reduce the circuit variations and make aircraft more similar to each other than one would expect. This, coupled with stable regulated power sources may allow for much faster reaction times or trip curves for AFCI devices designed for aerospace applications. In addition, 400 Hz AC power used in modern aircraft allows for more waveform comparisons in a given period of time: standard 60 Hz NEMA devices are designed to detect and arc fault in 7 cycles of power, (116.7 ms), at 400 Hz this takes only 17.5 ms. The increase of frequency coupled with more stable power, fixed loads, etc. indicate the devices should be well suited to prevent the electrical ignition source of aircraft fires. In the future, these devices may be board mounted in avionics power supplies and/or placed at individual electrical loads. They can be designed to communicate with one another or with data recorders to monitor the condition of electrical wiring and components. Maintenance data recorders can be reviewed after flight and pending failures identified and maintenance interventions can take place prior to system failure.
Laboratory tests have shown that AFCI breakers can detect faults not detectable by approved military aircraft circuit breakers and are significantly faster at detecting arcing faults in aircraft wiring.
Experiments were performed at International Aero Inc. with Schneider Electric, Square D Company to determine the differences between aircraft breakers and AFCI devices. These tests were based on the FAA Wet Arc Testing protocols developed to determine susceptibility of aircraft wire to arcing.
A five ampere rated (5A) Mil-Spec aircraft circuit breaker was placed in series with a fifteen ampere Square D Company Arc-D-Tect, AFCI, modified to operate at 400 Hz. Power was applied to an aircraft water boiler drawing 1.95 amps through the subject breaker and AFCI device. Arcs in the range of 75-100 amps were induced into the input to the boiler by dragging a 20 ga wire between input to the boiler to ground. In every test, the prototype AFCI interrupted the power before the Military-Standard aircraft breaker. These experiments indicate these devices can be adapted for use in aircraft AC circuits. Additional tests are ongoing to determine the detection differences with modified AFCI devices and standard aircraft circuit breakers, as well as the susceptibility of thermal acoustic insulation material to ignition from electrical arcs, and the ability of AFCI to mitigate the ignition.
There are two types of arcing faults in aircraft electrical circuits and wiring: Parallel and Series.
Parallel arcing occurs when there is an arc between two wires or wire-to-frame and the current is limited by the impedance of the voltage source, the wire, and the arc. When the fault is solidly connected and the arc voltage low, the normal aircraft breaker trips very quickly with little heating of the wire or damage at the arc point. Occasionally, however, the arc blows apart the faulted components creating a larger arc voltage and reducing the fault current below the trip curve and causing xe2x80x9cticking faults.xe2x80x9d The consequences of parallel arc damage, are usually much greater than series arcs. The average current may not be sufficient to trip a conventional breaker by heating the bimetal strip or the peak current may not be large enough to trigger the magnetic trip latch. This makes the Mil-Std breaker reasonably effective in protecting against parallel arcing when the peak current is a few hundred amps. Unfortunately, the fault current can be limited by a circuit with too much impedance to immediately trip the thermal-magnetic breaker. Parallel arcing is generally more hazardous than series arcing. The energy released in the arc is much higher with temperatures often in excess of 10,000 Deg. F. This causes pyrolyzation or charring of the insulation, creating conductive carbon paths and ejecting hot metal that is likely to encounter flammable materials.
Series arcing begins with corrosion in pin-socket connections or loose connections in series with the electrical loads. The voltage drop across a poor connection begins at a few hundred millivolts and slowly heats and oxidizes or pyrolizes the surrounding materials. The voltage drop increases to a few volts at which time it becomes a xe2x80x9cglowing connectionxe2x80x9d and begins to release smoke from the surrounding polymer insulation. Series arc current is usually limited to a moderate value by the impedance of the electrical load that is connected to the circuit. The amount of power from series arc is typically far is less than in a parallel fault. Since the peak current is typically never greater than the design load current, series arcing is much more difficult to detect than parallel arcing. The signature of the series arc is an unusual variation of the normal load current. Series arcing is usually such that the arc current remains well below the trip curve of the Mil-Spec aircraft breaker. Loose terminal lugs, misarranged or cross-threaded electrical plugs, broken conductor strands inside a wire are typical sources. These arcs cause load voltage drops and heating of the wire, plug pin, or terminal lug. This heating can lead to component failure and ignition source. Direct Current (DC) arcs are another serious event that can potentially be prevented with AFCI technology. DC loads are relatively stable and any changes designed into a circuit tend to be well documented with known load profiles. Changes in the DC circuit signature should be detectable even faster than those in AC circuits. Without the sinusoidal changes in voltage and polarity as seen in AC power, changes in a DC circuit should be detected even more reliably than AC circuits.
Care needs to be taken in the adaptation of AFCI into aerospace. Critical and essential electrical circuits need protection which will not nuisance trip. Most aircraft electrical loads are on branched circuits which provide a mixture of current waveforms to the breaker. A single breaker in the cockpit may feed several unrelated systems. Nuisance tripping is not acceptable as several systems may be powered by one breaker. Careful analysis should be used in design and implementation of AFCI technology in aerospace. Even with these reservations, AFCI has the potential to be one of the single largest improvements to aircraft safety in years.
Summarizing briefly, heat, arcs or electrical ignition are often caused by loose connections, broken or shorted wires in the power distribution system. In aircraft wiring, vibration, moisture temperature extremes, improper maintenance and repair all contribute to wiring failure. This leads to arcing and may ignite combustible components. Furthermore, carbon tracking caused by heat generated by the arc can deteriorate the wire insulation, exposing the conductors and resulting in intermittent short circuits between individual wires. These inter-wire shorts can cause damage to delicate avionics and cause system malfunctions in-flight. Elimination or reduction of these hazards to flight with arc fault technology should become an industry-wide priority.
The invention includes an apparatus and method by which arcing is detected in aircraft wiring.
Detection of the above-described sputtering currents caused by arcing is one object of the present invention. A detection signal generated in accordance with the invention can be used to trip a circuit breaker, to indicate arcing to the avionics package, to alert the pilot, or to issue a command to release a control relay.
It is an object of the present invention to provide an arc fault detection system and method which reliably detects arc fault conditions which may be ignored by conventional circuit interrupters.
Another object of the invention is to provide an arc fault detection system which utilizes a minimum number of highly reliable electronic signal processing components, such as a microcontroller, to perform most of the signal processing and analyzing functions, so as to be relatively simple and yet highly reliable in operation.
Other and further objects and advantages of the invention will be apparent to those skilled in the art from the present specification taken with the accompanying drawings and appended claims.
In accordance with one aspect of the invention, there is provided a method of determining whether arcing is present in an aircraft electrical circuit comprising the steps of sensing a current in said circuit and developing a corresponding input signal, determining the presence of broadband noise in said input signal, and producing a corresponding output signal, and processing said input signal and said output signal in a predetermined fashion to determine whether an arcing fault is present in said circuit.
In accordance with another aspect of the invention, there is provided a system for determining whether arcing is present in an aircraft electrical circuit comprising a sensor for sensing a current in said circuit and developing a corresponding sensor signal, a circuit for determining the presence in the sensor signal of broadband noise, and producing a corresponding output signal, and a controller for processing said sensor signal and said output signal in a predetermined fashion to determine whether an arcing fault is present in said circuit.
In accordance with another aspect of the invention, there is provided a controller for determining whether arcing is present in an aircraft electrical circuit in response to input signals, said input signals corresponding to a current in said circuit and to the presence of broadband noise in a predetermined range of frequencies in said circuit, said controller including a plurality of counters and wherein said controller increments said plurality of counters in a predetermined fashion in accordance with said input signals and periodically determines whether an arcing fault is present based at least in part on the state of said plurality of counters.
In accordance with another aspect of the invention, there is provided a method of determining whether arcing is present in an aircraft electrical circuit by processing input signals corresponding to a current in said circuit and to the presence of broadband noise in a predetermined range of frequencies in said circuit, said method comprising the steps of incrementing a plurality of counters in a predetermined fashion in accordance with said input signals, and periodically determining whether an arcing fault is present based at least in part on the state of said plurality of counters.
In accordance with another aspect of the invention, there is provided an electrical fault detector for aircraft wiring which comprises a first band-pass filter circuit responsive to an input signal representative of an electrical signal condition in a circuit to be monitored, which passes a frequency signal comprising signal components of said input signal which fall within a first predetermined frequency band and AND circuit means which receives and ANDs the frequency signals from the first and second band-pass filter circuits.
In accordance with another aspect of the invention, there is provided an application specific integrated circuit which comprises a first band-pass filter circuit responsive to an input signal representative of a signal condition in a circuit to be monitored which passes a frequency signal comprising signal components of said input signal which fall within a first predetermined frequency band, a second band-pass filter circuit means responsive to said input signal which passes a frequency signal comprising signal components of said input signal which fall within a second predetermined frequency band, and AND circuit which receives and ANDs said frequency signals from said first and second band-pass filter circuits.